It has long been the practice in both the textile and paper industries to apply starch solutions to the fibers for various purposes. The ultimate properties of such products have been improved by crosslinking the starch with polyfunctional compounds, such as glyoxal, and the like. With the advent of heat-hardenable resins such as urea and melamine resins, it became desirable to mix such resins with the starch to obtain an even more durable finish. Ultimately, heterocyclic reaction products of (i) alpha, beta dicarbonyl compounds, (ii) urea, thiourea or guanidine and (iii) an aldehyde, e.g., formaldehyde, assumed an important position in the art of textile finishing. In Richardson, U.S. Pat. No. 2,661,312, for example, a stable, curable finish for textiles comprises 1,3-bis-(hydroxymethyl)-2-imidazoline, starch and tartaric acid. The heterocyclic compound is made, for example, by reacting ethyleneurea with formaldhyde and has two nitrogen-bonded methylol groups which are capable of cross-linking the starch, when heated, the rate of cross-linking being promoted by the presence of tartaric acid. Important state-of-the-art textile finishes also have evolved from such technology. In v. Reibnitz, U.S. Pat. No. 2,764,573, condensation products of glyoxal and ureas, thioureas or guanidines (glyoxal monoureins) are modified by reaction with aldehydes, e.g., formaldehyde, or aldehydes and an alcohol, and there are produced the corresponding N-substituted alkylol or alkoxyalkyl substitutents. The resins are shown to cure, especially after the addition of acid hardening catalysts, to waterproof and elastic films. It has subsequently been discovered and is known in the art that the glyoxal monourein and aldehyde condensation products are of great importance when used to treat textiles. In Gagliardi et al, U.S. Pat. No. 3,209,010, it is disclosed that such materials, especially when further substituted on the 4- and 5-positions by ether, ester, carbamoyl groups, and the like, provide chlorine-resistant finishes on textiles.
The crosslinking of polyhydroxyl compounds, particularly polysaccharides like starch, with multifunctional reagents reactive with hydroxyl groups is known outside of the textile field, such as in the manufacture of paper board from wood and other fibers and foundry molds from sand.
Foundry cores and molds present unique problems. These are used in making metal castings and are normally prepared from a composition including sand or other refractory material and a curable or polymerizable binder coated on the refractory particles. The purpose of this binder coating is to permit the mixture to be hardened after it is first shaped or molded into a desired form. Shaping of the composition, which usually comprises a major amount of sand and a minor amount of binder, is accomplished through ramming, blowing, or otherwise introducing the mixture into a pattern or core box to thereby assume the shape defined by the adjacent surfaces of the pattern. Then, by using a catalyst or polymerization accelerator introduced before or after the sand mix has been introduced into the pattern, and/or by using heat, the binder is caused to cure, thereby converting the shaped foundry mix into a hard, solid foundry core. This curing is usually accomplished either in the original core box, in a gassing chamber, or in a holding pattern. Commonly used binders include such materials as phenolic resins, urea-formaldehyde resins, furfural alcohol modified urea-formaldehyde resins, furan resins, drying oils and urethane oils.
Generally speaking, two basic techniques exist in the art for effecting a cure once the sand-binder mixture is shaped. The first of these techniques, the elevated temperature method, involves the use of heatcurable resin system wherein heat is used to effect hardening of the binder. The second technique is known in the art as the "no-bake" or "cold-setting" process. As its name implies, the latter process is carried out at room temperature or slightly above, i.e., 5.degree.-50.degree. C. and more often between 15.degree.-35.degree. C.
Each of these systems has its own set of limitations which are well known to those active in the field. Some materials are very energy intensive; some pose significant handling and environmental problems; some have limited utility because gas evolution from the binder during metal pouring creates surface defects in the finished metal article; and, if the cores are to be baked, green strength additives must be used so that the cores have sufficient strength to be put into and through an oven.
In Cummisford et al., U.S. Pat. Nos. 4,013,629; 4,089,691; 4,098,615; 4,098,859 and 4,158,574, are disclosed the use of the catalyzed glyoxal saccharide system in foundry sand cores, cellulose press formed products, adhesives, coating binders and in many other areas. The developments in these patents overcome many problems by controlling the amounts of reactants and catalyst and by selecting the saccharide from the wide range of materials available. In practice, however, the ultimate products are somewhat deficient in hydrolytic stability, which limits the use of these binder systems to foundries in which the temperature and humidity can be controlled.
Further representative of the state of the art are Nishikawa et al, U.S. Pat. No. 4,482,654 who coat foundry sand grains with a binder comprised of methylolmelamine or alkylated methylolmelamine and a water soluble polyol, e.g., hydroxymethyl cellulose. Sand molds having a water soluble binder containing sucrose, urea, methylolmelamine or alkylated derivatives and an acidic crosslinking catalyst are disclosed in Japanese Patent Publication No. 59,185,542, Oct. 22, 1984, Chem. Abs. Vol 102:99473r (1985); binders for molding sand consisting of methylolmelamine precondensed with a saccharide are disclosed in French Patent Publication, Apr. 11, 1980 Chem. Abs. Vol 96:108797m (1982); and glucose mixed with methylated methylolmelamine and used as a binder for foundry sand is described in Japanese Patent Publication No. 57,124,542, Aug. 3, 1982; Chem. Abs. Vol 98:7323q (1983). All such systems show mainly hydrolytic stability problems common to the starch-glyoxal system mentioned above and there still exists the need for improved binder resins.
It has now been discovered, and is the subject of this invention, that binders prepared from polysaccharides and the condensation products of alpha, beta diketones, or alpha, gamma-diketones, urea or a sulfur or nitrogen analog thereof, or an N-substituted derivative thereof, optionally further condensed with an aldehyde or an aldehyde and an alcohol have remarkably beneficial properties. It is important, especially when using such binders with non-alkaline sands, to include an acid crosslinking catalyst in the composition.